SaintVicious, jungler for the CLG.NA team, once said it on stream. If you are certain that you are not going to be counterjungled, then sure, smite right away. However, circumstances can quickly change while you are attacking the creep (assuming smite is used on objectives like buffs).
Situation #1: You are jungling but leave base a little late. Like usual you head to Red and you feel confident so tell your team you don't need their help leashing it. So you smite right away and begin autoattacking it. In the animation of your last auto-attack, the enemy jungler smites it away from you over the wall, then turns and kills you with the help of the small lizards and the red buff they now have cuz your smite was down.
Situation #2: You have been jungling and cleared the jungle already. As typical, you get ready to feed your AP Carry the blue buff. While, waiting for the AP Carry to deal the finishing blow, the whole enemy team comes charging from river to the Blue led by their jungler. Fortunately this time, you saved your smite, so before the enemy jungler can smite steal it, you secure blue for your team. While not optimal, certainly better than the enemy team having both blue buffs.
Finally, it is good practice as it helps you use smite to secure important objectives like dragon and baron. After all practice makes perfect, and if you say that you always use smite first except for dragon and baron, you won't be as practiced. A few seconds lost on the smite cooldown is hardly noticeable most of the time, but gaining a little more practice on using your smite to secure kills never hurts anyone.
I've found a cost analysis of the core build vs the others, and a little what to expect.
http://www.rtsguru.com/game/435/article/3066/Gold-Items-In-Mid-Lane-A-CostBenefit-Analysis.html
The point is that buying up some gold per 10 items is a highly situational tactic. The most important element to determine when considering these items is your personal ability to farm. In the case of being an incredibly powerful farmer (pro players) or being an incredibly bad farmer (new players, low ELO players) these items either help you snowball more or counteract your poor farming skills. If you’re decent at last hitting (say, 50%) then you’ll hit an odd middle-ground where they are still okay items, but you may be better off getting one only one Doran’s and saving up for those more impactful items. Beware this valley and the triple gold per 10 build may just become your best friend in the middle lane.
And this one also gives a nice cost analysis, but also tells you abit about which champions and what to expect :
http://www.solomid.net/guides.php?g=27381
Best Answer
Lane priority means, you have priority to leave the lane and help somewhere else by having control over the lane. That means, if you can push the lane into the opponents tower, without them being able to stop it, you have lane control and thus priority.
This gives you several advantages:
However be careful, if you have lane priority, you also often play quite a bit forward and thus are easier to gank, which is why the jungler often plays around lanes, which have control.